Magnetic recording medium

ABSTRACT

A magnetic recording medium comprising a nonmagnetic support having provided thereon a magnetic layer comprising a ferromagnetic powder dispersed in a binder, wherein said ferromagnetic powder has a specific surface area of 40 m 2  /g or more as measured by the BET method; said magnetic layer further comprises Al 2  O 3  particles having an average particle diameter of from about 0.1 to 0.5 μm; and said binder comprises the combination of (1) a vinyl chloride resin containing a phosphate ester group represented by ##STR1## wherein M 1  and M 2 , which may be the same or different, each represents Na, K, Li, H, N⊕R 3 , or N⊕HR 2  wherein R represents an alkyl group or a hydrogen atom, and (2) at least one resin other than said vinyl chloride resin containing at least one of --SO 3  M, --OSO 2  M, --COOM, and ##STR2## wherein M, M 1 , and M 2 , which may be the same or different, each represents Na, K, Li, H, N⊕R 3 , or N⊕HR 2  wherein R represents an alkyl group or a hydrogen atom; provided that the weight ratio of said resin other than said vinyl chloride resin to said vinyl chloride resin is from about 0.4 to 2.5.

FIELD OF THE INVENTION

The present invention relates to a magnetic recording medium and, morespecifically, it relates to a magnetic recording medium which isexcellent in the reduction of the friction coefficient at the surface ofthe magnetic layer and in the high frequency output, and in whichdeterioration of S/N (signal/noise) ratio is prevented.

BACKGROUND OF THE INVENTION

It has been attempted to increase the packing density of magnetic powderby introducing hydrophilic functional groups to a binder for use in amagnetic recording medium. Particularly, in the case of a vinyl chloridetype resin, functional groups such as --OH and --COOH groups areintroduced for increasing the packing density of the dispersing matters.However, if the vinyl chloride type resin alone is used as the binder,the performance of the medium is remarkably degraded in view of therunning stability and the running durability because of its extremelypoor flexibility. In view of the above, resins excellent in flexibilitysuch as a polyurethane resin, a polyester resin, a phenoxy resin, anacrylonitrile rubber, etc. are used as the second ingredient forproviding the magnetic recording medium with flexibility.

The above resins not containing functional groups have been used as thesecond ingredient. However, since still higher density has been requiredfor magnetic recording media in recent years along with development inshort wavelength recording systems, acidic groups (such as carboxylic,sulfonic, sulfate, and phosphate groups), amino groups, imino groups,imido groups, amido groups, hydroxy groups, alkoxy groups, thiol groups,halogen groups, silyl groups, etc. have now been included in the secondingredient. Thus, the packing density of the magnetic powder can beimproved so as to be suitable for the short wavelength recording system.

However, if the dispersibility is increased excessively, the surfacesmoothness of the magnetic recording medium is also increased, and thedrawback exists that the friction coefficient is undesirably increased.Accordingly, it is difficult at present to improve the dispersibility onone hand while suppressing the increase in the friction coefficient dueto the mirror-smooth surface on the other hand.

Further, it has been intended to decrease the particle size offerromagnetic powder used for the magnetic recording medium to permitshort wavelength recording in recent years. In this circumstances, aproblem such as reduction in the high frequency output and deteriorationof the S/N ratio have been the result which are the problems encounteredat present.

For reducing the friction coefficient, surface active lubricants, forexample, fatty acids, fatty acid amides, fatty acid alcohols, fatty acidesters, or silicone oils, etc. have been used. Although the frictioncoefficient can be reduced to some extent by such means, it is not yetsatisfactory for ensuring sufficient running performance and runningdurability. Further, if they are added in an excess amount,disadvantages such as head contamination occur due to exudation of thelubricants, and this can not be considered an effective means.

In increasing the density of magnetic recording media in recent years,their friction coefficient tends to be increased by making the magneticparticles finer, increasing the packing density, and providing amirrorsmooth surface. However, both satisfactory running stability andsatisfactory electromagnetic conversion properties are difficult toattain by the mere application of the surface active agents, carbonblack, or inorganic powders. Further, there has been a great problem inrecent years that the high frequency output and the S/N ratio of themagnetic medium running in a deck are deteriorated.

In order to avoid these problems, method have been proposed in whichpoler groups, such as a sulfonate group, a sulfate group, a carboxylategroup, a phosphate group, etc., are introduced into the binder, but theyare still insufficient in view of the electromagnetic conversionproperties (as described in U.S. Pat. Nos. 4,529,661, 4,521,486, and4,613,545).

The present inventors have made earnest studies on such problems and, asa result, have found that if the frictional resistance between thesurface of the magnetic recording medium and the glass portion at theperiphery of a magnetic head is high, destruction is caused at thesurface of the glass portion by the high speed sliding contact of themedium and, as result, fine protrusions are formed on the glass surfaceand roughen the glass surface. It has been found that the fineprotrusions grow together with the running time, which increase thespacing loss between the magnetic surface of the medium and the magnetichead to result in the reduction in the high frequency output and the S/Nratio. It has have also been found that this phenomenon is greatlyinfluenced particularly by the grain size of the ferromagnetic powderand remarkably occurs in fine magnetic particles having a specificsurface area of 40 m² /g or more as measured by the BET method.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a magnetic recordingmedium having satisfactory electromagnetic conversion characteristicsand excellent running stability, and running durability, by combining aresin having hydrophilic functional groups with an alumina powder havingan appropriate grain size.

Other objects of the present invention will be apparent from thefollowing description.

The present inventors have studied the kind of functional groups of theresin and the powder added to the magnetic recording medium forovercoming the foregoing problems and, as a result, have found that boththe increase in the density of the magnetic recording medium and thesurface smoothness, and the reduction in the friction coefficient of themedium surface can be attained.

It has now been found that the above objects of the present inventioncan be attained by a magnetic recording medium comprising a nonmagneticsupport having provided thereon a magnetic layer comprising aferromagnetic powder dispersed in a binder, wherein the ferromagneticpowder has a specific surface area S_(BET) of 40 m² /g or more asmeasured by the BET method; the magnetic layer further comprises Al₂ O₃particles having an average particle diameter of from about 0.1 to 0.5μm; and the binder comprises the combination of (1) (the firstingredient) a vinyl chloride resin containing a phosphate ester grouprepresented by ##STR3## wherein M₁ and M₂, which may be the same ordifferent, each represents Na, K, Li, H, N⊕R₃, or N⊕HR₂ wherein Rrepresents an alkyl group or a hydrogen atom, and (2) (the secondingredient) at least one resin other than the vinyl chloride resincontaining at least one of --SO₃ M, --OSO₂ M, --COOM, and ##STR4##wherein M, M₁, and M₂, which may be the same or different, eachrepresents Na, K, Li, H, N⊕R₃, or N⊕HR₂ wherein R represents an alkylgroup or a hydrogen atom; provided that the weight ratio of the resinother than the vinyl chloride resin (the first ingredient) to the vinylchloride resin (the second ingredient) is from about 0.4 to 2.5.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is now described in more detail below.

According to the present invention, the vinyl chloride resin containingthe phosphate ester group as the first ingredient is excellent in thedispersibility not only of the magnetic powder but also, particularly,of Al₂ O₃, and the magnetic properties of the medium are notdeteriorated if Al₂ O₃ is added. Further, when the vinyl chloride resincontaining the phosphate ester group is combined with the functionalgroup-containing resin as the second ingredient and used together withAl₂ OH₃ having a suitable grain size, the frictional resistance to theglass portion of the magnetic head can be reduced remarkably and, as aresult, deterioration in the high frequency output and the S/N ratioafter running as described above can be improved. This is demonstratedas is shown by examples described later by the actual measurement usinga video tape and by measuring the friction coefficient with a glass rod.

The vinyl chloride resin containing the phosphate ester group used asthe first ingredient in the present invention is prepared as a polymeror a copolymer of phosphate ester group-containing monomers.

Vinyl chloride units provide, in cooperation with other constituentunits, an appropriate hardness and flexibility to the magnetic layer.Since the abrasion resistance of the magnetic recording medium becomespoor if the amount of vinyl chloride units is too small, whereas thesolvent solubility tends to be reduced if it is excessive, the vinylchloride content of the vinyl chloride resin is preferably from about 60to 95 wt%, and more preferably from 80 to 95wt%.

The vinyl monomer containing the phosphate ester group in the presentinvention is a polymerizable vinyl monomer substituted with a phosphateester group, and examples thereof include acid phosphoxy ethylacrylate,acid phosphoxy ethylmethacrylate, acid phosphoxy propylacrylate, acidphosphoxy propylmethacrylate, 3-chloro-2-acid phosphoxy propylacrylate,and 3-chloro-2-acid phosphoxy propylmethacrylate.

The phosphate ester group-containing vinyl monomer greatly contributesto the improvement of the dispersibility of the magnetic powder. If theamount of the phosphate group-containing monomer is too large, thesolubility of the copolymer to solvents is lowered failing to obtain auniform solution, the water resistance is deteriorated, and further,since side reactions other than the cross-linking reaction occurs withisocyanate compounds, the disadvantages exists that no cross linkingeffect for the improvement of the abrasion resistance can be obtained,and moreover the pot life of the magnetic coating composition isshortened. On the other hand, if the amount of the phosphategroup-containing monomer is too small, the dispersibility of themagnetic powder tends to be deteriorated. Accordingly, the content ofthe phosphate group-containing monomer in the vinyl chloride resin ispreferably from about 0.05 to 8%, and more preferably from 0.1 to 5% byweight.

The vinyl chloride resin containing phosphate group can be obtained bythe known polymerization processes, for example, precipitationpolymerization, solution polymerization, suspension polymerization,emulsion polymerization, etc. In the case of precipitationpolymerization, methanol or n-hexane which provides good solubility forthe vinyl monomer containing phosphate group, and in which the resultantpolymer is insoluble, is preferably used as a polymerization solvent,and methanol is particularly preferred in view of the solvent solubilityof the resulting copolymer and from an economic point of view. The vinylchloride resin can be obtained as a fine powder in any of the solvents.

The polymerization degree of the vinyl chloride resin is preferably fromabout 150 to 600, more preferably from 200 to 450, in view of themechanical strength and the adhesiveness of the magnetic layer and thecharacteristics of the magnetic coating composition for the magneticlayer. The resin having an average degree of polymerization of less than150 is unsuitable since it provides only a weak coated surface when itis applied to a support, whereas the resin having a degree ofpolymerization of more than 600 increases the viscosity of the coatingcomposition to reduce the workability upon coating.

The functional group-containing resin other than vinyl chloride as thesecond ingredient in the present invention can include, for example, anacrylate/acrylonitrile copolymer, an acrylate/vinylidene chloridecopolymer, an acrylate/styrene copolymer, a methacrylate/acrylonitrilecopolymer, a methacrylate/ vinylidene chloride copolymer, amethacrylate/styrene copolymer, a urethane elastomer, a nylon-siliconeresin, a nitrocellulose-polyamide resin, a polyvinyl fluoride resin, avinylidene chloride/acrylonitrile copolymer, a butadiene/acrylonitrilecopolymer, a polyamide resin, a polyvinyl butyral, cellulose derivatives(cellulose acetate butyrate, cellulose diacetate, cellulose triacetate,cellulose propionate, nitrocellulose, etc.), a styrene/butadienecopolymer, a polyester resin, a chlorovinyl ether/acrylate copolymer, anamino resin, thermoplastic resins of various kind of synthetic rubbersand mixtures thereof. Among these, a urethane elastomer, a vinylidenechloride/acrylonitrile copolymer, a butadiene/acrylonitrile copolymer,cellulose derivatives, and a polyester resin are preferred, and aurethane elastomer, a butadiene/acrylonitrile copolymer, and a polyesterresin are most preferred.

In the present invention, at least one of --SO₃ M, --OSO₂ M, --COOM, and##STR5## wherein M, M₁, and M₂, which may be the same or different, eachrepresents Na, K, Li, H, N⊕R₃, or N⊕HR₂ wherein R represents an alkylgroup or a hydrogen atom is contained in the non-vinyl chloride resin asthe functional groups.

Acidic groups are preferred as the functional groups in view of the potlife and the functional groups described above are particularlyexcellent in view of the dispersibility.

In the present invention, the weight ratio of the second ingredient tothe first ingredient is preferably from about 0.4 to 2.5, morepreferably from 0.4 to 1.5, and most preferably from 0.4 to 1.0. If theratio is less than about 0.4, the magnetic recording medium has aninsufficient flexibility resulting in disadvantages such as dropping ofpowder in view of the running durability. On the other hand, if itexceeds about 2.5, the medium becomes too soft and causes runningtroubles such as adhesion.

The binder in the present invention may further contain otherpolymerizable monomers such as ethylene, propylene, vinyl acetate, etc.as a comonomer of the vinyl chloride resin, and the homopolymers orcopolymers of these copolymerizable monomers may be used together withthe first and second ingredients. In the case of these copolymerizablemonomer is a comonomer of the vinyl chloride resin, the amount thereofis preferably from 1 to 20 wt% based on the amount of the vinyl chlorideresin. In the case where the polymer or copolymer of thesecopolymerizable monomer is used, the amount of such polymer or copolymeris preferably from 0.5 to 25 wt% based on the total amount of the firstand second ingredients. Particularly, use of ethylene as a comonomerunit for the vinyl chloride polymer is preferred in view of theimprovement in the solvent solubility.

In the present invention, a polyisocyanate may preferably be usedtogether with the first and second ingredients. The added amount thereofis preferably from 0.2 to 56. 25 wt%, and more preferably from 0.25 to56.25 wt% based on the total amount of the first and the secondingredients.

The polyisocyanate usable in the present invention can includeisocyanates, for example, tolylene diisocyanate, 4,4'-diphenylmethanediisocyanate, hexamethylene diisocyanate, xylylene diisoicyanate,naphthylene-1,5-diisocyanate, o-tuluidine diisocyanate, isophoronediisocyanate, and triphenylmethane triisocyanate; reaction products ofthese isocyanates and polyalcohols; and polyisocyanates formed by thecondensation of isocyanates. Among these compounds, tolylenediisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylenediisocyanate, and isophorone diisocyanate are preferably used in thepresent invention. These polyisocyanates are commercially availableunder the trade names of, for example, Collonate L, Collonate HL,Collonate 2,030, Collonate 2,031, Millionate MR, Millionate MTL(manufactured by Nippon Polyurethan Co.), Takenate D-102, TakenateD-110N, Takenate D-200, Takenate D-202 (manufactured by Takeda YakuhinCo.), Desmodur L, Desmodur IL, Dismodur N, Dismodur HL (manufactured bySumitomo Bayer Co.), etc., which may be used alone or in a combinationof two or more of them utilizing the difference in curing reactivity.Further, a compound having a hydroxy group or an amino group may be usedtogether to promote the curing reaction. It is desirable that thesecompounds are polyfunctional.

The ferromagnetic fine powder usable in the present invention caninclude known ferromagnetic fine powders such as γ--Fe₂ O₃,Co-containing γ--Fe₂ O₃, Fe₃ O₄, Co-containing Fe₃ O₄, γ--FeO_(x),Co-containing γ--FeO_(x) wherein 1.33≦×1.50, CrO₂, Co-Ni-P alloy,Co-Ni-Fe-B alloy, Fe-Ni Zn alloy, Ni-Co alloy, Co-Ni-Fe alloy, etc.Specifically, they are described, for example, in Japanese PatentPublication Nos. 14090/69, 18372/70, 22062/72, 22513/72, 28466/71,38755/81, 4286/72, 12422/72, 17284/72, 18509/72, 18573/72, 10307/64,29280/73, 39639/73, 29605/83, 44254/85, Japanese Patent Application(OPI) No. 126605/84, U.S. Pat. Nos. 3,026,215, 3,031,341, 3,100,194,3,242,005, and 3,389,014. In addition, hexagonal tabular barium ferritemay also be used. The term "OPI" used herein means a publishedunexamined Japanese patent application.

It is a further feature of the present invention that the ferromagneticpowder has a specific surface area S_(BET) of about 40 m² /g or more,preferably from 40 to 80 m² /g, as measured by the BET method.

The ratio of the amount of the ferromagnetic powder to the total amountof the magnetic layer at the dry state is preferably from 0.2 to 0.69 byweight, more preferably from 0.3 to 0.69 by weight.

Dispersants, lubricants, antistatic agent, etc. may be dissolved in thesolvent and adsorbed on the surface of the ferromagnetic fine powderprior to the dispersion for the respective purposes described later.

In the present invention, the magnetic layer contains Al₂ O₃ powderhaving an average grain size (particle diameter) from about 0.1 to 0.5μm as an abrasive agent. The conventional magnetic recording mediumgenerally contains a combination of 1 to 4 kinds of abrasive agentshaving Moh's hardness of 6 or more such as α-alumina, molten alumina,silicon carbide, chromium oxide, cerium oxide, corundum, artificialdiamond, α-iron oxide, garnet, emery (mainly composed of corundum andmagnetite), silica rock, silicon nitride, boron nitride, molybdenumcarbide, boron carbide, tungsten carbide, titanium carbide, tripoli,diatomaceous earth, dolomite, etc. Among these inorganic powders, whenAl₂ O₃ powder having an average grain size of from about 0.1 to 0.5 μmis used in combination with the binder as described above, the advantageof remarkably reducing the friction coefficient and smoothing thesliding movement with the glass portion is obtained. If the averagegrain size is less than about 0.1 μm, the effect of reducing thefriction coefficient is insufficient and, on the other hand,satisfactory electromagnetic conversion properties can not be maintainedif it is more than about 0.5 μm. The Al₂ O₃ powder is preferably addedto the magnetic recording medium in an amount of from about 1 to 19parts per 100 parts by weight of the ferromagnetic powder. The effect ofthe present invention can be attained if it is used in combination withother abrasive agents than Al₂ O₃.

In the present invention, additives such as carbon black, a lubricant, adispersant, an antistatic agent, etc. can further be incorporated intothe magnetic layer.

In the present invention, carbon black can be incorporated into themagnetic layer and furnace carbon black for rubber, thermal carbon blackfor rubber, color carbon black, or acetylene carbon black can be used asthe carbon black. Specific example of abbreviation of the carbon blacksin the U.S. include SAF, ISAF, IISAF, T, HAF, SPF, FF, FEF, HMF, GPF,APF, SRF, MPF, ECF, SCF, CF, FT, MT, HCC, HCF, MCF, LFF, RCF, etc. andthose classified in ASTM Standard D-1765-82a can be used. The carbonblack used in the present invention preferably has an average grain sizeof from about 5 to 1,000 mμ (as determined by an electron microscope), aspecific surface area of from about 1 to 800 m² /g determined by thenitrogen adsorption method, a pH value of from about 4 to 11 (accordingto JIS K-6221-1982), and a dibutylphthalate (DBP) oil adsorption amountof from 10 to 800 ml/100 g (according to JIS K-6221-1982). The grainsize of the carbon black used in the present invention is preferablyfrom 5 to 100 mμ for reducing the surface electric resistance of thecoated film, and from 50 to 1,000 mμ for controlling the strength of thecoated film. Further, the carbon black of fine particles (100 mμ orless) can be used for smoothing to reduce spacing loss, and carbon blackof coarse grains (50 mμ or more) is used with an aim of roughening thesurface to reduce the friction coefficient. Thus, the kind and theaddition amount of the carbon black used are dependent on the purposerequired for the magnetic recording medium.

Further, the carbon black may be used after the surface treating withthe dispersant described later, or being grafted with resins. Further,these carbon blacks partially graphitized at the surface thereof bytreatment at a temperature of 200° C. or more in a furnace uponproducing carbon black may also be used. Furthermore, hollow carbonblack may specially be used. It is preferred that the carbon black isused in an amount from about 0.1 to 20 parts by weight per 100 parts byweight of the fine ferromagnetic powder in the magnetic layer. Carbonblacks which can be used in the present invention are described inCarbon Black Binran (Manual for Carbon Black), edited by Carbon BlackAssociation (1972).

The lubricant usable for the magnetic layer in the present invention caninclude, for example, silicone oil, graphite, molybdenum disulfide,boron nitride, graphite fluoride, fluorinated alcohol, polyolefin (e.g.,polyethylene wax, etc.), polyglycol (e.g., polyethylene oxide wax,etc.), alkyl phosphate ester, polyphenyl ether, tungsten disulfide, afatty acid ester formed of a monohydric fatty acid ester having from 10to 20 carbon atoms and one or more of mono-valent, di-valent,tri-valent, tetra-valent and hexa-valent alcohols having from 3 to 12carbon atoms, and fatty acid esters formed of mono-basic fatty acidshaving from 10 or more carbon atoms and monovalent to hexa-valentalcohols having a number of carbon atoms providing the total number ofcarbon atoms of from 11 to 28. Fatty acids, fatty acid amides, and fattyacid alcohols each having from 8 to 22 carbon atoms may also be used.Specific examples of the organic compound lubricants include butylcaprylate, octyl caprylate, ethyl laurate, butyl laurate, octyl laurate,ethyl myristate, butyl myristate, octyl myristate, ethyl palmitate,butyl palmitate, octyl palmitate, ethyl stearate, butyl stearate, octylstearate, amyl stearate, anhydrosorbitan monostearate, anhydrosorbitandistearate, anhydrosorbitan tristearate, anhydrosorbitan tetrastearate,oleyl oleate, oleyl alcohol, lauryl alcohol, etc.

As the lubricant for use in the present invention, conventionallubricant oil additives may also be used, and examples thereof includeanti-oxidants (alkyl phenole, etc.), rust inhibitors (naphthenic acid,alkenyl succinic acid, dilauryl phosphate, etc.), oilly agents (rapeseedoil, lauryl alcohol, etc.), extreme pressure agents (dibenzyl sulfide,tricresyl phosphate, tributyl phosphite, etc.), cleaning dispersants,viscosity index improvers, flow point depressants, defoaming agents,etc. These lubricants are preferably added in an amount of from about0.05 to 20 parts by weight per 100 parts by weight of the ferromagneticpowder. These agents are described e.g., in Japanese Patent PublicationNos. 238898/68, 4041/73, 18482/73, 18221/69, 28043/72, and 56132/82,U.S. Pat. Nos. 3,423,233, 3,470,021, 3,492,235, 3,497,411, 3,523,086,3,625,760, 3,630,772, 3,634,253, 3,642,539, 3,687,725 and 4,135,031, andIBM Technical Disclosure Bulletin vol. 9, No. 7, p 779 (December 1966).

The dispersant usable in the present invention can include fatty acidshaving from 10 to 22 carbon atoms such as caprylic acid, capric acid,lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid,elaidic acid, linoleic acid, linolenic acid, stearolic acid (R₁ COOH inwhich R₁ represents an alkyl group having from 9 to 21 carbon atoms),alkali metal soaps thereof (Li, Na, K, etc.) alkaline earth metal soapsthereof (Mg, Ca, Ba, etc.), Cu, Pb, etc. soaps of the fatty acidsdescribed above; lecithin, etc. In addition, higher alcohols having 4 ormore carbon atoms, such as butanol, octanol, myristyl alcohol, stearylalcohol, as well as sulfate esters and phosphate esters thereof may alsobe used. The dispersant is preferably added in an amount of from about0.005 to 20 parts by weight per 100 parts by weight of the binder. Thedispersant may be previously deposited on the surface of the fineferromagnetic powder or the fine non-magnetic powder, or added duringdispersing the coating composition. These agents are described, forexample, in Japanese Patent Publication Nos. 28639/64, 17945/69,18221/69, 39402/74, 15001/73, and U.S. Pat. Nos. 3,387,993 and3,470,021.

The antistatic agent usable in the present invention can include, forexample, an electroconductive powders such as graphite, carbon black,carbon black graft polymer; natural surface active agents such assaponin; nonionic surface active agents such as alkylene oxides,glycerins, glycidols, polybasic alcohols, polybasic alcohol esters,alkylphenol ethylene oxide addition products; cationic surface activeagents such as higher alkyl amines, cyclic amines, hydantoinderivatives, amide amines, ester amides, quarternary ammonium salts,pyridine and other heterocyclic compounds, and phosphoniums orsulfoniums; anionic surface active agents containing acidic groups suchas carboxylic, sulfonic phosphoric, sulfate, and phosphorate groups;amino acids; amphoteric surface active agents such as amino sulfonicacids, sulfuric acid or phosphoric acid esters of amino alcohols, andalkyl betaines. Examples of the surface active agent compound usable asthe antistatic agents are disclosed, for example, in U.S. Pat. Nos.2,271,623, 2,240,472, 2,288,226, 2,676,122, 2,676,924, 2,676,975,2,691,566, 2,727,860, 2,730,498, 2,742,379, 2,739,891, 3,068,101,3,158,484, 3,201,253, 3,210,191, 3,294,540, 3,415,649, 3,441,413,3,442,654, 3,475,174, 3,545,974, West German Patent Application (OLS)No. 1,942,665, British Patents 1,077,317 and 1,198,450, as well asRyohei Oda, Kaimennkasseizai no Gosei to sono Oyo (Synthesis andApplication of Surface Active Agent), (Maki Shoten 1972); A. W. BailySurface Active Agents (Inter Science Publication Corporated 1985);Encyclopedia of Surface Active Agents, vol. 2, (Chemical PublishingCompany 1964); Kaimenkasseizai Binran (Surface Active Agent Manual),(6th edition, Sagyo Tosho Co., Dec. 20, 1966); and Hideo Marushige,Taiden Boshizai (Antistatic Agent) (Miyuki Shobo 1968), etc.

These surface active agents may be added alone or in combination. Theamount of the surface active agent used in the magnetic recording mediumof the present invention is preferably from about 0.01 to 10 parts byweight per 100 parts by weight of the fine ferromagnetic powder. Theseagents are used as an antistatic agent, but may often be used for otherpurposes, for example, improvement in dispersibility, magneticproperties, and lubricating properties, and as coating assistants. Theseagents are described, for example, in Elektronik, Vol. 9 No. 7, page 779(1961); ibid No. 12, page 380 (1961); Kagaku Binran (Manual ofChemistry), application section, pages 954 to 967, (Maruzen Co. 1980).

The organic solvent used in the present invention, used for dispersing,kneading, and coating, can include ketones such as acetone, methyl ethylketone, methyl isobutyl ketone, cyclohexanone, and isophorone; alcoholssuch as methanol, ethanol, propanol, butanol, isobutanol, isopropanol,and methyl cyclohexanol; esters such as methyl acetate, ethyl acetate,butyl acetate, isobutyl acetate, isopropyl acetate, and ethyl lactateglycol monoethylether acetate; ethers such as diethyl ether,tetrahydrofuran, glycol dimethyl ether, glycol monoethyl ether anddioxane; tar type aromatic hydrocarbons such as benzene, toluene,xylene, cresol, chlorobenzene, and styrene; chlorinated hydrocarbonssuch as methylene chloride, ethylene chloride, carbon tetrachloride,chloroform, ethylene chlorohydrin, and dichlorobenzene; N,N-dimethylformaldehyde; and hexane.

There is no particular restriction in the present invention for themethod of kneading and dispersing, and the order of adding each of theingredients is not limited. The magnetic coating composition can beprepared by using a conventional kneader, for example, a 2-roll mill, a3-roll mill, a ball mill, a pebble mill, a tron mill sand grinder, aSzegvari attritor, a high speed impeller, a disperser, a high speedstone mill, a high speed impact mill, a disper, a kneader, a high speedmixer, a ribbon blender, a kneader, an intensive mixer, a tumbler, ablender, a disperser, a homogenizer, a single-axis screw extruder, adouble-axis screw extruder, and a ultrasonic wave disperser. Details ofthe techniques of kneading and dispersing are described, for example, inT. C. Patton Paint Flow and Pigment Dispersion (John Wiley & Sons 1964),Kogyo Zairyo (Industrial Material) Vol. 25, 37 (1977) by ShinichiTanaka, and publications cited therein. They are further described inU.S. Pat. Nos. 2,581,414 and 2,855,156. In the present invention, themagnetic coating composition can be prepared by kneading and dispersingin accordance with the methods as described in these publications.

The magnetic recording layer may be formed by optionally selecting theingredients as described above, dissolving and dispersing them in anorganic solvent, and coating as a coating composition on a supportfollowed by drying. The dry thickness of the magnetic layer ispreferably from about 1.5 to 7.0 μm, and more preferably from 3.0 to 6.0μm. In the case of preparing a magnetic tape, it is preferred that thethickness of the support is from about 2.5 to 100 μm, and preferablyfrom about 3 to 70 μm. In the case of a disk or a card-like material,the thickness of the support is preferably from about 0.5 to 10 mm, andin the case of a drum, it may used in a cylindrical shape. As thematerial for the support, there can be used polyesters such aspolyethylene terephthalate, polyethylene naphthalate; polyolefins suchas polypropylene, cellulose derivatives such as cellulose triacetate andcellulose diacetate; vinyl resins such as polyvinyl chloride; otherplastics such as polycarbonate, polyamide, polysulfone, etc. as well asmetals such as aluminum and copper, and ceramics such as glass. Thesesupports may be subjected to corona discharging treatment, plasmatreatment, primer coating, heat treatment, dustremoving treatment, metalvapor deposition, and alkali treatment prior to the coating. Thesesupports are described, for example, in West German Patent No.,3338854A, Japanese Patent Application (OPI) No. 116926/84, U.S. Pat. No.4,388,368; and Sen'i to Kogyo (Fibers and Industry), vol. 31, pages 50to 55, by Yukio Mitsuishi (1975).

As the method of coating the coating composition the magnetic recordinglayer onto the support, there can be used air doctor coating, bladecoating, air knife coating, squeeze coating, dip coating, reverse rollcoating, transfer roll coating, gravure coating, kiss coating, castcoating, and spray coating, as well as other methods. These methods arespecifically described e.g., in Coating Kogaku (Coating Technology),pages 253 to 277 (Asakura Shoten, Mar. 20, 1971).

After the magnetic layer is coated on a support by such a method, themagnetic powder in the layer is magnetically oriented by a conventionalmethod as required while drying, and then the formed magnetic layer isdried. The conveying speed for the support in this case is usually fromabout 10 to 900 m/min and the drying temperature is usually controlledto from about 20° to 30° C.

The magnetic recording medium according to the present invention isprepared by further applying surface smoothing techniques or cuttinginto a desired shape as required. These processes are described, forexample, in Japanese Patent Publication 23635/65 and 28368/64, and U.S.Pat. No. 3,473,960, Further, the methods described in Japanese PatentPublication 13181/67 which is a fundamental and important techniques inthe relevant field of art can also be applied to the present invention.

The fine ferromagnetic powder, the non-magnetic powder, the binders, theadditives, the solvents, and the supports, (optionally including aprimer layer, a backing layer, a backing primer layer) and theproduction processes for the magnetic recording medium described inJapanese Patent Publication No. 26890/81 can be applied to the presentinvention.

The present invention will now be described in more detail withreference to specific example, but the present invention is not to beconstrued as being limited thereto. Unless otherwise indicated, allparts, percents, and ratios are by weight.

EXAMPLES 1 AND 2 AND COMPARATIVE EXAMPLES 1 TO 8

Each of the example composition was prepared as follows.

After charging a portion of the composition described below into a ballmill followed by sufficiently kneading, the remaining portion of thecomposition was charged in the ball mill and sufficiently kneaded. Then,15 parts of Desmodur L75 (trade name of polyisocyanate compound,manufactured by Bayer Co.) were added thereto and uniformly mixed anddispersed to prepare a magnetic coating composition.

    ______________________________________                                        Co-containing γ-Fe.sub.2 O.sub.3 powder                                                           100    parts                                        Vinyl chloride resin (fist ingredient)                                                                  10     parts                                        Non-vinyl chloride resin (second ingredient)                                                            10     parts                                        Al.sub.2 O.sub.3 powder   5      parts                                        Carbon black              5      parts                                        Lecithin                  1      part                                         Oleic acid                1      part                                         Octyl laurate             1.5    parts                                        Butyl acetate             200    parts                                        Methyl ethyl ketone       100    parts                                        ______________________________________                                    

In Example and Comparative Examples, the kind of Co-containing γ--Fe₂ O₃powder, vinyl chloride resin, resin other than vinyl chloride (non-vinylchloride resin), and Al₂ O₃ powder in the composition were varied asfollows.

EXAMPLE 1

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 45 m² /g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing phosphate ester groups

Functional group content: 6.5×10⁻⁵ eq/g

Number average molecular weight: 16,000

Polymerization degree: 380

Carboxyl-containing polyurethane (TiM-3005, manufactured by Sanyo KaseiCo.)

Functional group content: 1×10⁻⁵ eq/g

Number average molecular weight: 22,000

Al₂ O₃ powder

Average grain size: 0.2 μm

EXAMPLE 2

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 45 mμ/g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing phosphate ester group(same as in Example 1)

Sulfonic group-containing polyurethane (UR-8300, manufactured by ToyoBoseki Co.)

Number average molecular weight: 25,000

Functional group content: 3.4×10⁻⁵ eq/g

Al₂ O₃ powder

Average grain size: 0.2 μm

EXAMPLE 8

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 45 m² /g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing phosphate ester group

(same as in Example 1)

Carboxyl group-containing polyurethane (TiM-3005, manufactured by SanyoKasei Co.)

(same as in Example 1)

Cr₂ O₃ powder

Average grain size: 0.5 μm

COMPARATIVE EXAMPLE 1

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 45 m² /g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing carboxyl group(400×110A, manufactured by Nippon Zeon Co.)

Functional group content: 1×10⁻³ eq/g

Number average molecular weight: 15,000

Polymerization degree: 420

Polyurethane (N-2304, manufactured by Nippon Polyurethane Co.)

Functional group content: none

Number average molecular weight: 34,000

Al₂ O₃ powder

Average grain size: 0.2 μm

COMPARATIVE EXAMPLE 2

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 35 m² /g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing carboxyl group(400×110A, manufactured by Nippon Zeon Co.)

(same as in Comparative Example 1)

Polyurethane (N-2304, manufactured by Nippon Polyurethane Co.)

(same as in Comparative Example 1)

Al₂ O₃ powder

Average grain size: 0.2 μm

COMPARATIVE EXAMPLE 3

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 30 m² /g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing carboxyl group(400×110A, manufactured by Nippon Zeon Co.)

(same as in Comparative Example 1)

Polyurethane (N-2304, manufactured by Nippon Polyurethane Co.)

(same as in Comparative Example 1)

Al₂ O₃ powder

Average grain size: 0.2 μm

COMPARATIVE EXAMPLE 4

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 45 m² /g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing phosphate ester group

(same as in Example 1)

Polyurethane (N-2304, manufactured by Nippon Polyurethane Co.)

(same as in Comparative Example 1)

Al₂ O₃ powder

Average grain size: 0.2 μm

COMPARATIVE EXAMPLE 5

Co-containing --Fe₂ O₃ powder

Specific service area (BET method): 30 m² /g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing phosphate ester group

(same as in Example 1)

Carboxyl group-containing polyurethane (TiM-3005, manufactured by SanyoKasei Co.)

(same as in Example 1)

Al₂ O₃ powder

Average grain size: 0.2 μm

COMPARATIVE EXAMPLE 6

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 45 mμ/g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer containing phosphate ester group

(same as in Example 1)

Carboxyl group-containing polyurethane

(TiM-3005, manufactured by Sanyo Kasei Co.)

(same as in Example 1)

Al₂ O₃ powder

Average grain size: 0.06 μm

COMPARATIVE EXAMPLE 7

Co-containing γ--Fe₂ O₃ powder

Specific service area (BET method): 45 m² /g

Coersive force Hc: 880 Oe

Vinyl chloride-vinyl acetate copolymer

containing phosphate ester group

(same as in Example 1)

Carboxyl group-containing polyurethane (TiM-3005, manufactured by SanyoKasei Co.)

(same as in Example 1)

Al₂ O₃ powder

Average grain size: 0.7 μm

In Examples and Comparative Examples, the electro-magnetic conversioncharacteristics on a VHS deck and the friction coefficients to glass andstainless steel were measured by the following procedures.

Video S/N

The difference between the S/N ratio of the sample tape and that of"Super AGT 120" manufactured by Fuji Photographic Film Co. as thestandard tape was determined by using a noise meter (925C) manufacturedby Shibasoku. The noise level was measured with a high pass filter at10KHz and low pass filter at 4MHz. The VTR used was "NV-8300"manufactured by Matsushita Co.

Video S/N degradation

The difference between the video S/N after one minute running on theabove deck and that after one hour running on the above deck wasmeasured.

Friction coefficient to a stainless steel pole

The sample video tape and a stainless steel pole were brought intocontact under a tension of 50 g (T₁), and the tension (T₂) required forrunning the tape at a speed of 3.3 cm/sec under this condition wasmeasured. The friction coefficient μ of the video tape which is shown inTable 1 was determined by the following formula:

    μ=1/π·1nT.sub.2 /T.sub.1

Friction coefficient to a glass pole

The friction coefficient to a glass pole was determined in the samemanner as the friction coefficient to the stainless steel pole asdescribed above.

The results of the measurement are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                             Friction coefficient                                                 Video S/N ratio                                                                        to stainless                                                                            Friction coefficient                       Sample No.                                                                           Video S/N ratio                                                                        degradation                                                                            steel pole (μ)                                                                       to glass pole (μ)                       __________________________________________________________________________    Example 1                                                                            +3.5 (dB)                                                                              -0.4 (dB)                                                                              0.25      0.20                                       Example 2                                                                            +3.3     ±0    0.26      0.20                                       Comparative                                                                          +2.0     -0.8     0.33      0.32                                       Example 8                                                                     Comparative                                                                          +3.2     -2.2     0.45      0.40                                       Example 1                                                                     Comparative                                                                          +2.0     -0.7     0.35      0.29                                       Example 2                                                                     Comparative                                                                          +1.3     ±0    0.27      0.20                                       Example 3                                                                     Comparative                                                                          +2.6     -0.3     0.27      0.22                                       Example 4                                                                     Comparative                                                                          +2.4     ±0    0.26      0.21                                       Example 5                                                                     Comparative                                                                          +3.7     -2.5     0.42      0.45                                       Example 6                                                                     Comparative                                                                          +1.2     -0.3     0.22      0.20                                       Example 7                                                                     __________________________________________________________________________

EXAMPLE 4

A magnetic coating composition having the formulation shown below wasprepared and coated on a nonmagnetic support made of polyethyleneterephthalate having a 10 μm thickness in the same manner as in Example1 so that the thickness of the magnetic layer after drying was 3.0 μm.

    ______________________________________                                        Ferromagnetic Fe--Ni alloy powder                                                                       100    parts                                        Ni content: about 5 wt %                                                      specific surface area                                                         determined by BET method: 45 m.sup.2 /g                                       Vinylchloride-vinylactate copolymler                                                                    10     parts                                        containing phosphate ester group                                              (same as in Example 1)                                                        Sulfonic group-containing polyurethane                                                                  8      parts                                        (UR-8300, manufactured by Toyo Boseki)                                        (same as in Example 2)                                                        Carbon black (Vulcan XC-72,                                                                             1      part                                         manufactured by Cabot Co.)                                                    grain size: 32 mμ                                                          Al.sub.2 O.sub.3 powder (Average grain size: 0.2 μm)                                                 8      parts                                        Oleic acid                0.5    parts                                        Myristic acid             1.5    parts                                        Octyl laurate             3      parts                                        Methyl ethyl ketone       200    parts                                        Polyisocyanate (Collonate L,                                                                            10     parts                                        manufactured by Nippon Polyurethan)                                           ______________________________________                                    

Magnetic field orientation was applied to the nonmagnetic support coatedwith the magnetic coating composition when the coating composition wasnot yet dried, and after drying and calendering to the surface, it wasslit to 8 mm widths to prepare a 8 mm video tape.

COMPARATIVE EXAMPLE 9

A tape was prepared in the same manner as in Example 4 except forreplacing the vinylchloridevinyl acetate copolymer containing phosphoricacid ester group with a vinylchloride-vinyl acetate copolymer containingcarboxyl group (VMCH, manufactured by Union Carbide Co., number averagemolecular weight: 21,000, polymerization degree: 340, functional group(--COOH) content : 9.5×10⁻⁵ eq/g).

The sample tapes obtained in Example 4 and Comparative Example 9 weretested as follows.

Video S/N

The difference between the S/N ratio of the sample tape and that of"Super AGP 6 90" manufactured by Fuji Photographic Film Co. as thestandard tape was determined by using a noise meter (925C) manufacturedby Shibasoku. The noise level was measured with a high pass filter at10KHz and low pass filter at 4MHz. The VTR used was "FUJIX Z 600AF"manufactured by Fuji Photo Film Co., Ltd.

Video S/N degradation

The difference between the video S/N ratio of the initial first pass andthat after 10 times pass running was measured.

Friction coefficient to stainless steel and glass poles

The sample video tape and each of a stainless steel pole and a glasspole were brought into contact under a tension of 20 g (T₁), and thetension (T₂) required for running the tape at a speed of 1.4 cm/secunder this condition was measured. The friction coefficient of the videotape was determined by the Following formula:

    μ=1/π·1nT.sub.2 /T.sub.1

The results of the evaluation for the 8 mm video tapes prepared inExample 4 and Comparative Example 9 are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                     Friction                                                                              Friction                                                    Video S/N coefficient                                                                           coefficient                                       Video S/N ratio     to stainless                                                                          to glass                                 Sample No.                                                                             ratio     degradation                                                                             Steel pole                                                                            pole                                     ______________________________________                                        Example 4                                                                              +0.5 dB   -0.6 dB   0.23    0.25                                     Comparative                                                                            -0.5 dB   -3.5 dB   0.27    0.39                                     Example 9                                                                     ______________________________________                                    

Apparent from the results shown in Tables 1 and 2, it is possibleaccording to the invention to obtain a magnetic recording medium havingsatisfactory electromagnetic conversion characteristics, with nodeterioration in the high frequency output and the S/N ratio, andexcellent in running stability.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A magnetic recording medium comprising anonmagnetic support having provided thereon a magnetic layer comprisinga ferromagnetic powder dispersed in a binder, wherein said ferromagneticpowder has a specific surface area of 40 m² /g or more as measured bythe BET method; said magnetic layer further comprising Al₂ O₃ particleshaving an average particle diameter of from about 0.1 to 0.5 μm; andsaid binder comprising the combination of (1) a vinyl chloride resincontaining a phosphate ester group represented by ##STR6## wherein M₁and M₂, which may be the same or different, each represents Na, K, Li,H, N⊕R₃, or N⊕HR₂ wherein R represents an alkyl group or a hydrogenatom, and (2) at least one resin other than said vinyl chloride resincontaining at least one of --SO₃ M, --OSO₂ M, --COOM, and ##STR7##wherein M, M₁, and M₂, which may be the same or different, eachrepresents Na, K, Li, H, N⊕R₃, or N⊕HR₂ wherein R represents an alkylgroup or a hydrogen atom; provided that the weight ratio of said resinother than said vinyl chloride resin to said vinyl chloride resin isfrom about 0.4 to 2.5.
 2. A magnetic recording medium as claimed inclaim 1, wherein the vinyl chloride content of said vinyl chloride resinis from about 60 to 95 wt%.
 3. A magnetic recording medium as claimed inclaim 2, wherein the vinyl chloride content of said vinyl chloride resinis from 80 to 95 wt%.
 4. A magnetic recording medium as claimed in claim1, wherein the content of the phosphate group-containing monomer in saidvinyl chloride resin is from about 0.05 to 8% by weight.
 5. A magneticrecording medium as claimed in claim 4, wherein the content of thephosphate group-containing monomer in said vinyl chloride resin is from0.1 to 5% by weight.
 6. A magnetic recording medium as claimed in claim1, wherein said vinyl chloride resin has a degree of polymerization offrom about 150 to
 600. 7. A magnetic recording medium as claimed inclaim 6, wherein said vinyl chloride resin has a degree ofpolymerization of from 200 to
 450. 8. A magnetic recording medium asclaimed in claim 1, wherein the weight ratio of said resin other thansaid vinyl chloride resin to said vinyl chloride resin is from about 0.4to 1.5.
 9. A magnetic recording medium as claimed in claim 8, whereinthe weight ratio of said resin other than said vinyl chloride resin tosaid vinyl chloride resin is from 0.4 to 1.0.
 10. A magnetic recordingmedium as claimed in claim 1, wherein said binder further comprises anisocyanate.
 11. A magnetic recording medium as claimed in claim 1,wherein said magnetic layer comprises from about 1 to 19 parts by weightof said Al₂ O₃ particles per 100 parts by weight of said ferromagneticpowder.
 12. A magnetic recording medium as claimed in claim 1, whereinsaid ferromagnetic powder has a specific surface area of from 40 to 80m² /g as measured by the BET method.
 13. A magnetic recording medium asclaimed in claim 1, wherein the weight ratio of the amount of saidferromagnetic powder to the total amount of said magnetic layer is from0.2 to 0.69.
 14. A magnetic recording medium as claimed in claim 13,wherein the weight ratio of the amount of said ferromagnetic powder tothe total amount of said magnetic layer is from 0.3 to 0.69.
 15. Amagnetic recording medium as claimed in claim 1, wherein saidferromagnetic powder is Co-containing 8--Fe₂ O₃ powder having a specificsurface area S_(BET) of about from 40 to 80 m² /g as measured by the BETmethod.
 16. A magnetic recording medium as claimed in claim 15, whereinsaid ferromagnetic powder is Co-containing 8--Fe₂ O₃ powder having aspecific surface area S_(BET) of about from 45 to 80 m² /g as measuredby the BET method.
 17. A magnetic recording medium as claimed in claim1, wherein said magnetic layer further comprises carbon black having aprimary particle size of from 20 to 120 μm in an amount of from 0.1 to15 parts by weight per 100 parts by weight of said ferromagnetic powder.18. A magnetic recording medium as claimed in claim 17, wherein saidcarbon black has a primary particle size of from 20 to 120 μm in anamount of from 0.5 to 12 parts by weight per 100 parts by weight of saidferromagnetic powder.